1. Technical Field
This application relates generally to half-bridge circuits employing normally on switches and to electrical devices which include one or more such circuits.
2. Background of the Technology
Silicon Carbide (SiC), a wide band-gap semiconductor material, is very attractive for use in high-power, high-temperature, and/or radiation resistant electronics. SiC power switches are logical candidates for these applications due to their excellent material physical properties such as wide energy band-gap, high breakdown field strength, high saturated electron drift velocity and high thermal conductivity compared to the conventional silicon counter part. In addition to the above advantages, SiC power devices can operate with much lower specific on-resistance than conventional silicon power devices [1]. Because of these properties, SiC unipolar devices are expected to displace silicon bipolar switches (such as the insulated gate bipolar transistor or IGBT) and rectifiers in the 600-3000 V range.
A necessary component to the value proposition that the market demands to adopt the new SiC switch technology is found in the lower specific on resistance of SiC switches. A factor of ten or more reduction is inherent to SiC devices, but only in the form most likely to produce these specific on-resistance reductions. The SiC junction field effect transistor (JFET) has demonstrated the lowest specific on-resistance of any SiC switch. In particular, the normally on version of this device will minimize the specific on-resistance at any voltage rating. The market place, however, tends to prefer normally off devices because they are perceived as being inherently safe when imbedded in an application.
An independent trend in the market place is to package powerful switches into modules that typically consist of two, four, or six switches arranged in parallel half-bridge configurations. These modules are particularly useful for the control of rotating machinery in devices known as motor drives, the operating principals of which are well known [2, 3]. FIG. 1 illustrates a single half-bridge module containing two silicon IGBT switches. Typical ratings for the switches in the modules are 600-V or 1200-V blocking potential and hundreds of amperes of conduction (e.g., 300-A, 400-A, or 600-A) but the voltages and currents can be higher or lower. The half-bridge interconnection and the constituent IGBT both have weaknesses. The former requires the added complication of an isolated gate driver for the high-side switch. The latter has a gate terminal that can be easily damaged by incorrect or excessively stressful control potentials. The latter also has many other failure modes which designers must avoid when designing applications, principally by making design tradeoffs in the circuitry known as the gate drive. The market has responded by including imbedded gate driver circuitry into the switch modules so that the application external to the module does not actually apply potentials to the gate terminals of the IGBTs. This additional level of safety has dramatically increased the reliability of IGBTs used in the field.
There still exists a need, however, for improved circuits and devices which modulate the flow of electrical current from one or more electrical sources to one or more electrical loads.